Programming Positive Mechanofluorescence in Liquid Crystalline Elastomers

Liquid single crystal elastomers (LSCEs) containing organic fluorophores within their polymeric network are attractive materials to detect forces with simple spectroscopic measurements. Hitherto, all mechanoluminescent LSCEs decrease their emission intensity upon mechanical stimulation; that is, they display negative mechanofluorescence. Such behavior is governed by the mechanically induced approximation of the quenching mesogenic units and the fluorophores. In this work, we propose the integration of fluorescent molecular rotors (FMRs), whose luminescence is not quenched by the mesogens, in LSCEs as a valuable strategy to conceive elastomeric materials programmed with exactly the opposite behavior, i.e., their fluorescence increases upon deformation (positive mechanofluorescence). Specifically, carbazole-indolenine dyes are interesting candidates for this purpose since their luminescence depends mainly on the degree of intramolecular rotation allowed by the local environment. On this basis, the uniaxial deformation of an LSCE, along its anisotropic direction, incorporating such FMRs will place the fluorophores in a more restricted medium, leading to the desired enhanced emission at the macroscale.

Materials and methods.Chemicals were purchased from commercial sources and used as received with the exception of THF, CH2Cl2 and DMF.THF was distilled over sodium and benzophenone under a nitrogen atmosphere.CH2Cl2 was distilled over CaH2 under a nitrogen atmosphere.Commercially available anhydrous DMF was stored over activated 4 Å molecular sieves under a nitrogen atmosphere.Thiophene-free toluene was prepared by washing commercially available toluene with concentrated sulfuric acid until the acid layer was colorless.The organic layer was washed twice with water, once with a solution of potassium carbonate (10% w/w), again with water, dried over anhydrous CaCl2 and finally distilled through an efficient column.Thiophene-free toluene was stored over activated 5 Å molecular sieves.All reactions were monitored by thin-layer chromatography using silica gel 60 F254 plates (Merck) and visualized under UV light (254 nm or 366 nm).Column chromatography was performed over silica gel (VWR, 40-63 µm).NMR spectra were recorded with a Varian Mercury 400 spectrophotometer.
Chemical shifts (δ) were referred to the residual solvent signal.High-resolution mass spectra (HRMS) were recorded with a LC/MSD-TOF Agilent Technologies spectrometer by means of the electrospray ionization (ESI) technique.
The crude was diluted with a saturated aqueous solution of NaHCO3 and the product was extracted with dichloromethane.The combined organic extract was dried over anhydrous Na2SO4, filtered and the solvent was evaporated under reduced pressure.The crude was purified by flash column chromatography using a mixture of hexane and ethyl acetate (8:2 v/v) as eluent to afford Cbz-db-In-OMe (48%, 1.969 g, 4.99 mmol). 1
Determination of the fluorescence quantum yields (Φf) was performed using quinine sulfate in 0.1 M HClO4 (Φf = 0.60 at 20-40 ºC) as a standard. [3]The excitation wavelength was 365 nm and the emission spectra were recorded in the range of 380 -600 nm.The absorption of the distinct solutions was kept below 0.10 to prevent inner filter effects.The quantum yields were calculated using equation ( 1): [4]  , =  , where Φf is the quantum yield, F is the integrated fluorescence intensity, Abs is the absorbance of solution at the excitation wavelength, n is the refractive index of the solvent.The subscripts x and st stand for the sample and standard, respectively.The measurements were triplicated.

Preparation of the LSCEs.
A solution of the distinct monomers, i.e.M4OMe (89% mol), CL (10% mol) and the corresponding fluorophore (Cbz-sb-In-C6 or Cbz-db-In-C6, 1% mol), and polyhydrogenomethylsiloxane (~85 Si-H groups per chain, AB101031, purchased from abcr) in thiophene-free toluene (1 mL) was placed in a Teflon mould.A solution of cyclooctadieneplatinum (II) chloride in CH2Cl2 (1% w/w, 40 µL) was added and the reaction mixture was heated in an oven at 75 ºC for 30 minutes.Then, the mold was cooled down to room temperature and the elastomer (not totally cross-linked) was carefully removed from the mold.During the deswelling process, a uniaxial force was applied to the hung elastomer, parallel to its longest axis, in order to achieve a macroscopic orientation of the nematic directors.After, the cross-linking reaction was completed by leaving the elastomer under load in an oven at 75 ºC for 2 days.The non-S-9 reacted monomers were removed from the network by a swelling-deswelling process using toluene and hexane, respectively.
Characterization of the LSCEs.DSC thermograms were recorded with a Mettler-Toledo DSC821 calorimeter at a scan rate of 10 ºC•min −1 under a nitrogen flow.Polarized optical microscopy (POM) was carried out at room temperature using a Nikon Eclipse polarizing microscope.POM experiments were run by rotating the analyzer of the microscope with respect to the longest axis of the elastomeric sample.X-ray scattering experiments were performed in a PANalytical X'Pert PRO MPD / powder diffractometer (radius = 240 mm) with a PIXcel detector (active length = 3.347º) in a convergent beam configuration and a transmission geometry.All LSCEs were sandwiched between low absorbing polyester films (thickness = 3.6 μm).X-Ray scattering patterns were registered at room temperature with the monochromatic Cu Kα radiation (λ = 1.5418Å) at an operating power of 45 kV (40 mA).Slits were adjusted in such a way that the height of the resulting incident beam was equal to 400 μm.A mask to define a beam length, in the axial direction, of about 4 mm was also used.2/ scans were registered from 2 = 1º to 2 = 60 º with a step size of 2 = 0.026º and a measuring time of 300 s per step.On the other hand, all azimuthal scans were collected at a step size of  = 1º and a measuring time of 2.55 s per step.From the azimuthal distribution of intensities at a scattering vector corresponding to the maximum of the reflex located at 2θ = 19.7−20.0º,which follows a Gaussian function, the angular distribution of the mesogens with respect to the director can be calculated.The order parameter, S, has been determined using the method reported by Lovell and Mitchell. [5,6]In the swelling experiments, the dimensions of the network in the deswollen and swollen state in toluene were measured at room temperature with a graduated magnifying glass.performed in a PTI 810 Series spectrophotometer (see above) controlled by a PC equipped with the PTI Felix32 software.All mechanofluorescent experiments were carried out by gluing the elastomeric samples by both ends into a self-constructed setup.LSCE samples were attached to the sample holder the previous day of the experiment to ensure thermodynamic equilibrium in the system.The sample was placed inside the spectrophotometer in a front-face geometry.In all instances, the fluorophores were excited at λEx = 365 nm; the resulting luminescence was collected from 400 to 600 nm.All experiments were carried out under ambient conditions.Once the first emission spectrum was collected a uniaxial deformation along the director direction of the LSCE was applied stepwise and the variation in the emission intensity was monitored from 400 to 600 nm.
After deformation, the elastomeric sample was left to equilibrate for a minimum of 15 minutes prior to each spectrum collection.The collected spectra were later transferred to a commercially available software (Origin version 2018) for further data treatment.

Figure S2 .
Figure S2.Absorption (a and b) and emission (c and d, λEx = 340 nm) spectra of 20 μM solutions of Cbz-sb-In-OMe (a and c) and Cbz-db-In-OMe (b and d) in different solvents.
Figure S3.X-ray diffraction pattern (a and b) and azimuthal intensity distribution of the wide angle reflex at 2θ = 19.7º(c and d, spacing = 4.5 Å) for the liquid single crystal elastomer E-sb-1 (a and c) and E-db-1 (b and d).